package string;

import java.util.Scanner;

public class test2 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        String str=sc.nextLine();
        int arr[]=new int[26];
        for (int i = 0; i <str.length() ; i++) {
            char ch=str.charAt(i);
            arr[ch-'a']++;
        }
        for (int i = 0; i <str.length() ; i++) {
            if(arr[i]==1){
                char ch=(char) (i+97);
                System.out.println(ch);
                break;
            }
        }

    }
}
